The present invention relates to a method of processing a high frequency signal containing at least two different fundamental frequencies and their harmonic components in order to extract at least one harmonic component of each of the two fundamental frequencies. In the present specification xe2x80x9chigh frequencyxe2x80x9d means above about 0.1 MHz.
The invention has particular application in the field of plasma processing and, more particularly, to the monitoring and control of plasma processes used in the fabrication of integrated circuits, optical and display devices, magnetic storage media and any other process in which a plasma is used to modify a film substrate.
In a typical plasma process reactor a high power radio frequency (RF) source, typically 0.1 to 100 MHz, is used to break down the gas in the reactor to form a conducting medium containing both electrons and ions. Often a second RF source is used to apply a bias voltage to the substrate being processed to aid, enhance or enable the surface reactions required by the process. In some applications the RF source used to form the plasma has a frequency F1 and the bias RF has a different frequency F2.
There is a significant electrical impedance mismatch between the plasma and the output of the RF generators. A matching unit reduces this mismatch. The matching unit can deal with changes in the plasma density (which alters the plasma impedance) by altering mechanical elements in the match unit. More recently, the matching unit does not have moving components but the match is maintained by altering F1 or F2 to maintain a match.
It has been noted in the prior art that by accurately measuring the RF voltage and current as close to the plasma chamber as possible, the plasma process user can better maintain and control the quality of the plasma process. This has significant financial impact by increasing yield and reducing faults.
Furthermore, the harmonic content of the frequency F1 or F2 can contain useful information on the state of a process, and can be used to detect end of process, or detect faults.
An RF voltage and current sensor, such as that described in U.S. Pat. No. 5,808,415, can be used to measure the RF current and voltage within the plasma chamber and provide corresponding signals Vi and Vv. However, where the plasma process is fed with two different frequencies F1 and F2 the signals Vi and Vv are composite signals containing components of both fundamentals F1 and F2 and their harmonics.
Therefore, there is a need to provide a method of accurately measuring the fundamental and harmonic components of both F1 and F2 in the composite RF signal Vi or Vv, even when F1 and F2 are variable. Current state of the art cannot achieve the rejection ratio and resolution to achieve this type of measurement.
According to the present invention there is provided a method of processing a high frequency signal containing at least two different fundamental frequencies F1 and F2 and their harmonic components in order to extract at least one harmonic component of each of said two fundamental frequencies, the method comprising sampling the signal at two sampling frequencies CK1=F1(M1/N1) and CK2=F2(M2/N2) where M1 and N1 are a first pair of integers having no common factor and M2 and N2 are a second pair of integers having no common factor, storing the sample values resulting from sampling at CK1 cyclically in a set of M1 memory locations such that the nth sample value is stored cumulatively in the remM1[n]th memory location, storing the sampled values resulting from sampling at CK2 cyclically in a set of M2 memory locations such that the nth sample value is stored cumulatively in the remM2[n]th memory location, continuing said sampling at each sampling frequency CK1 and CK2 until sufficient sample values have been stored in each memory location of the corresponding set M1 or M2 to substantially average out the other fundamental frequency(s) and its/their harmonic components, and performing a Fourier analysis of the said sample values in each set M1 and M2 of memory locations to extract the said at least one harmonic component of each fundamental frequency F1 and F2.
Using the invention, it is possible to sample current and voltage waveforms and measure accurately to very high resolution ( greater than 100 dB) the fundamental amplitude of current, voltage and phase of two or more frequencies F1 and F2 in a composite signal. It is also possible to measure accurately the current, voltage and phase of the harmonics of both F1 and F2, typically up to the fifth harmonic, in order to measure important events in the process such as end-point of the process.